1. Field of the Invention
The present invention relates to a correlator that determines the correlation between an OFDM signal, which has an effective symbol period and a guard interval in which part of the effective symbol signal has been copied, and a delay signal obtained by delaying the OFDM signal. The present invention also relates to a demodulation device that includes the correlator. In particular, the present invention relates to a correlator and a demodulation device including the correlator that can also cope with multipath propagation circumstances where the incoming time difference is large and the level difference (power ratio) is small.
2. Description of the Related Art
In recent years, the orthogonal frequency division multiplexing (OFDM) modulation method has been used as a modulation method in digital terrestrial broadcasting.
In the OFDM method, multiple subcarriers with different center frequencies are utilized to transmit symbols. Here, a symbol is a set of data transmitted in one modulation.
One symbol cycle is configured as a result of a guard interval (GI) being added to the effective symbol period. In the OFDM method, as shown in FIG. 7, part of the effective symbol signal serving as the actual demodulation target is copied and inserted between effective symbol signals as a repeated waveform. This allows OFDM to suppress the influence of multipath interference. The interval of this copied waveform is the guard interval.
In the case of demodulating this OFDM signal, the received OFDM signal is digitally converted by an A/D converter, the guard interval is removed, the effective symbol signal is extracted, and the effective symbol signal is demodulated by a fast Fourier transformer (FFT). Namely, as shown in FIG. 8, a correlation value between the received OFDM signal and the signal obtained by delaying the OFDM signal by the length of the effective symbol period is determined Additionally, a maximum value of a value obtained by integrating this correlation value is extracted, and, on the basis of the timing of this maximum value, the guard interval is removed, the effective symbol period is extracted, and the effective symbol period is demodulated by the FFT (e.g., see Japanese Patent Application Laid-Open (JP-A) No. 11-163824 and JP-A No. 2000-059332).
However, in a case where the received power is weak, or in a case where the influence of phasing and multipath is strong, or in a case where a narrowband noise signal has entered the reception band, sometimes the correlation value becomes small and the timing shifts or the timing becomes unable to be taken and the reception characteristic deteriorates.
In particular, a case where there is multipath will be described below. That is, a case where not only a signal incoming directly (main incoming path) but also a signal incoming slower than the main incoming path by reflection or the like (long delay path) come in on the reception side will be described.
In a conventional correlator, time-delayed received signals are not included at the time of single path reception (single path time) of only the main incoming path. For this reason, in a conventional correlator, an autocorrelation output showing one peak such as shown in FIG. 9A is obtained. However, at the time of multipath reception (multipath time) of the main incoming path and the long delay path, multiple peaks occur in accordance with incoming time differences. For example, FIG. 9B shows a two-path reception.
That is, with the conventional correlation output, the timing cannot be detected well because the correlation value becomes high the most in the time position of the main incoming path at the time of reception of only one path. However, at the time of two-path reception, a strong correlation appears in the incoming time positions of the main incoming path and the long delay path, so that, as shown in FIG. 9B, the correlation output signal resembles a trapezoid having two peaks in which there is a time difference of a peak interval equal to the delay time of the second path (called “long delay time” below). In actual transmission, the heights of these two peaks change because of the influence of interference power sections or the waveform of the OFDM signal. For this reason, in a case where a conventional correlator is used to generate time synchronization information on the basis of the maximum value of the peak, the position of the maximum correlation goes back and forth between the two time positions separated from each other by the long delay time. For this reason, in conventional correlation output, time synchronization become unstable, inter-symbol interference arises, and the reception characteristic ends up deteriorating.
Therefore, as a correlator that eliminates the above problem, for example, second and third embodiments of JP-A No. 2009-55204 discloses a correlator that can cope with this in consideration of multipath.
That is, in JP-A No. 2009-55204, the correlator employs five delay circuits, or a memory circuit, an address decoder, and a selector having the same functions as those, and three difference correlation circuits to add three correlation outputs whose time positions have been delayed and output the correlation outputs as one correlation signal. For this reason, in this technology, in a case where there exists a long delay path having the same received power as the path that becomes the main incoming wave, a strong correlation appears in the middle position of the main incoming path and the long delay path. Consequently, according to JP-A No. 2009-55204, it becomes possible to prevent a strong correlation from appearing in the incoming positions of the main incoming path and the long delay path and to suppress time synchronization instability.
However, particularly under multipath propagation circumstances where the incoming time difference is large and the level difference is small, the effect is insufficient even with the technology disclosed in JP-A No. 2009-55204.